Signals of sensors, which are used to detect physiological properties, cannot typically be used further directly, but must be filtered in a suitable manner. This serves especially the purpose of separating the desired useful signal, for example, an ECG signal, from undesired signals, e.g., noise or motion artifacts, in order to let thereby the useful signal proper stand out more clearly.
Various methods have long been known for carrying out such a filtering. These methods are based mostly on so-called linear filters, which can be described by linear mathematical transformations (e.g., Fourier transformation). Linear filters can be designed as comparatively simple filters and possess signal-independent properties, i.e., the properties do not depend on the signal being processed. Such filters are typically characterized by a so-called system function, which reflects the frequency response and the phase response.
However, any form of signal processing (“filtering”) is associated, in principle, with a certain time requirement. This means that the processed signal is available at the output of the signal-processing unit after a certain delay time only. The value of the delay time depends, on the other hand, very decisively on the filter properties.
However, this delay time must not exceed a certain limit in case of some applications, especially during the processing of physiological signals, because, e.g., a medical engineering device must change its characteristic fast enough to serve its purpose. For example, a respirator must be able to respond fast enough to the wish of a patient to be respirated, because respiration cannot otherwise be carried out adequately.
Since there are physiological signals, which require, on the one hand, a large amount of filtering, i.e., a high filter quality, but only short delay times are permissible, on the other hand, an especially high requirement on signal processing, i.e., on the filter being used, will arise here.
Linear filters reach their limits in a short time here, because, in particular, their ability to separate signals (“filter slope” or “bandwidth”) is linked directly with the delay time. Strictly speaking, there is a relationship between the settling time and the frequency bandwidth needed therefor. One speaks of time-bandwidth product here in the theory of filters. This means that the product of the settling time by the frequency bandwidth is constant. This relationship is fundamental and is inherently due to the linear properties of these filters. Therefore, there is no possibility to build narrow-band filters or filters having a steep cut-off with a very short delay time. Long settling times mean that the filter generates artifacts, which are of a long duration. In other words, any brief change in the input signal generates at the output a filter artifact of a long duration. This phenomenon is known under the term “transient response.” In other words, stated more simply, linear filters possess either poor properties in case of short delay times or good properties in case of long delay times.
Besides the classic linear filters, there are filters which, contrary to linear filters, can partly process time-variant signals and are partly based on nonlinear methods. Examples herefor are Blind Source Separation, Independent Component Analysis, Principal Component Analysis, Adaptive Filters and Empirical Mode Decomposition. Even though some of these filters possess considerably better properties than classic linear filters, the effort needed for signal processing that is associated with them is great, and longer or shorter delay times will always arise. Therefore, these filters cannot be used for all applications.
Therefore, only another type of nonlinear filters can be considered for these applications. In particular, the settling time or the duration of filter artifacts and the filter quality (“filter slope” or “bandwidth”) cannot be uncoupled from each other in nonlinear filters. One example of such a filter is a threshold value detector, which sets all values above a certain threshold, e.g., to zero. The time delay is obviously very short here, because a threshold value comparison can be performed very fast and, furthermore, no filter artifacts will appear. Consequently, there is no post-pulse oscillation in these filters.
Various filter solutions are known from the state of the art.
Thus, DE 42 35 318 C2 describes an apparatus for removing a baseline fluctuation from an ECG signal, in which a forward filter and a backward filter are connected in series via a buffer. An ECG signal is applied here to the forward filter, which has a nonlinear phase response characteristic and generates filtered signal data. These data are stored in the buffer, and blocks of the stored data are applied in reversed chronological order to the likewise nonlinear backward filter in order to generate blocks filtered in the backward direction in this manner.
DE 197 28 782 B4 pertains to a nonlinear filter for transducer signals having a vibration component for use in motor vehicles. The transducer signals are applied to an input of the filter, and a difference of an output signal of the filter from this input signal is formed in a differentiating member, and the difference thus formed is sent to an integrating member once directly and once via a nonlinear transmission member. The nonlinear transmission member is provided with a middle zone of relatively low gain, which has a variable width, with the width of the middle zone being adjusted by an amplitude detector, which forms the amplitude difference between the highest signal value and the lowest signal value of the filter input signal during each vibration period duration of the interfering vibration in the input signal, such that the time constant of the filter is reduced to a low value only during changes in the useful signal and the unwanted signal does not therefore appear at the filter output.
DE 195 18 528 A1 discloses a digital high-pass filter with means for restoring the baselines, wherein the high-pass filter is derived from a low-pass filter. The low-pass filter is formed by a first low-pass filter and a second low-pass filter, each of which receives a common input signal and outputs a corresponding output signal. The first filter has a relatively high limit frequency, so that it accurately follows the input signal, but does not significantly attenuate the ripple thereof. The second filter has a limit frequency that can be changed as a function of a control signal, which is generated on the basis of a comparison of the output signals of both filters.
EP 0 677 922 A2 pertains generally to a fast low-pass filter and a slow low-pass filter, wherein both filters receive the same input signal and the outputs of the two filters are compared in order to increase the response time of the slow low-pass filter if the difference of the outputs exceeds a certain value, so that the slow low-pass filter can follow the input signal faster.
EP 0 749 056 B1 shows a regenerative tracking filter using a plurality of integrators.
EP 1 346 743 A1 discloses a device for controlling a respirator with a breathing gas release control, which is connected to a sensor for a measured signal and builds up pressure as a function of a trigger signal corresponding to the measured signal.
EP 1 793 374 A1 pertains to a filter apparatus for active noise reduction.
U.S. Pat. No. 4,248,240 shows an apparatus for detecting the activity of the respiratory organs and of the heart of a living being, in which EMG signals are filtered by means of a high-pass filter, with which a comparator is connected in parallel.
U.S. Pat. No. 5,777,909 pertains to a low-pass filter with coefficient switchover for improving the settling time.
U.S. Pat. No. 7,535,859 B2 pertains to the problem of speech activity recognition with the use of adaptive tracking of the background noise.
US 2009/0143693 A1 generally describes an apparatus for generating determination indices in order to identify ECG unwanted signals.
US 2010/0168595 A1 pertains to a method and an apparatus for eliminating a baseline shift with a first amplifier and a second amplifier, between which a low-pass filter is inserted. The output of the first amplifier is sent to the low-pass filter as well as to a time delay circuit connected in parallel to the low-pass filter, whose output signal is added to the output signal of the low-pass filter and is sent to the second amplifier.
The following documents shall be mentioned as further prior art: AU 707148 B2; DE 101 64 446 A1; DE 10 2007 024 072 A1; DE 10 2007 062 214 B3; EP 0 889 291 A2; EP 1 365 296 A1; U.S. Pat. No. 4,915,103; U.S. Pat. No. 5,353,788; U.S. Pat. No. 5,980,463; U.S. Pat. No. 6,588,423; US 2004/0260186 A1; US 2006/0152197 A1; WO 98/48877; WO 2006/131149 A1; WO 2006/029529 A1; DE 199 59 822 A1; U.S. Pat. No. 5,820,560 and WO 2009/096820 A1.